Discharged 12 volt battery

[Big Ears]

On the advice of others here and other forums I bought this one.

We go to France next year for 6 weeks or more, It'll double up as a backup in the Motorhome for charging my mobility scooter I think (anyone help on that point please?) and it frees up the cabs ciggy sockets too, it has a torch and socket to charge phones, even an outlet that looks like it may charge a laptop.

https://amzn.eu/d/6MeP8jB

It does have other functions, watch the videos too for more info, it's a nice bit of kit as well.

 

[Joningy]

Probably a bit silly of MG to put the MG3 battery in the boot then.

Well, I didn't realise that Clive, yes indeed rather silly if the boot is not accessible like the MG4 range during 12V failure.

It was a general comment to try and help others, fortunately my MG4 Trophy LR has not suffered during two 4-week periods unused in an airport car park.

What I did forget to mention and its seems fairly important if correct, is to leave the traction High Voltage batteries over 50% charged whilst leaving the car for long periods, I remember reading somewhere, under 50% the High voltage traction battery no longer tops up the 12V, I’ve no idea if this is true, but it has so far worked for me, plus its easy to charge up to 80 +% near most airports before the flight, enough to get us home, I don’t want to be faffing around, often in the dark when we arrive at our home airport!

 

[Deleted member 18108]

Well, I didn't realise that Clive, yes indeed rather silly if the boot is not accessible like the MG4 range during 12v failure.

It was a general comment to try and help others, fortunately my MG4 Trophy LR has not suffered during two 4-week periods unused in an airport car park.

What I did forget to mention and its seems fairly important if correct, is to leave the traction High Voltage batteries over 50% charged whilst leaving the car for long periods, I remember reading somewhere, under 50% the High voltage traction battery no longer tops up the 12v, I’ve no idea if this is true, but it has so far worked for me, plus its easy to charge up to 80 +% near most airports before the flight, enough to get us home, I don’t want to be faffing around, often in the dark when we arrive at our home airport!

There is a lug in the fuse box under the bonnet for jump charging, to save the hassle of getting into the boot.

 

[Joningy]

This is the one I bought, and appears to be on offer at the moment:
https://www.amazon.co.uk/GOOLOO-Starter-Emergency-Portable-Starters/dp/B07S8B3TSY/ref=sr_1_2_sspa?crid=1V21NUA5FEUAS&dib=eyJ2IjoiMSJ9.XSLJQx5_dLrZWk8L0rPX02zZ0hgZWYo729zJEMKM26KS8n-IOwHaOcHYuM5K2gWwEy6KHZ2AuhBJaBnhRhBkyqsatfPVNg8ivKf3uSXU3tJlcrphnC_vpqJYPJd_EOJh8Tx984gMhfgiMdZPstDNFz3Fwe1tlWPQ62nRkfxp-dj5PxGDkimCAQjpWow0wqCBnVCKuM3bdi512GUL6ko1L9dOQ2Lf88aLY7NYbI8--_1trXTJQREfBDRoBSSfwNg-GlTPvbrW3RbvFU6BfeJeA2VQoVRRnj9NCyGGfF2Oq04.UNv8sfDZja3uHNHc5YsjUI0J36luk3lsfI8fuO3_wGk&dib_tag=se&keywords=GOOLOO+Jump+Starter+Power+Pack+Quick+Charge+Out+1500A+Peak+12V+Car+Battery+Booster&qid=1723370569&sprefix=gooloo+jump+starter+power+pack+quick+charge+out+1500a+peak+12v+car+battery+booster,aps,87&sr=8-2-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&th=1

HaHa, I guess the compass is useful if it's dark AND foggy, to help see which way the car is facing!

There is a lug in the fuse box under the bonnet for jump charging, to save the hassle of getting into the boot.

Hopefully that's in the basic user instructions, probably not a salesman's main selling point lol.

 

[MG Clive]

There is a lug in the fuse box under the bonnet for jump charging, to save the hassle of getting into the boot.

Thank you @DeclanB, I didn't know that.

** EDIT **
Just found it in the User Manual.

 

[MadBaz]

Probably a silly question (sure someone will tell me) where is the negative terminal if using the fuse box method ?

 

[Big Ears]

There should be a pin somewhere, but the whole chassis it an earth point as is the engine.

 

[MG Clive]

Probably a silly question (sure someone will tell me) where is the negative terminal if using the fuse box method ?

Possibly one of the other terminals next to the positive terminal in the fuse box, but I couldn't be sure. But you can attach the negative to any decent sized exposed bolt attached to the body/chassis under the bonnet.

 

[MadBaz]

Oops missed that , told you it was a daft question ?‍

There should be a pin somewhere, but the whole chassis it an earth point as is the engine.

Thanks for that ?

 

[Big Ears]

The electric boot opener is the most ridiculous thing, we've had a cable for decades that just worked, they must agree as that is how the bonnet and petrol flap work I think (not looked) Confucious say "flat battery mean" yoo going nowhere.

 

[TonyD 2024]

Car Jump Starter, FLYLINKTECH 2000A Peak 13200mAh Portable Car Battery Jump Starter Booster (up to 6.0L Gas Or 5.0L Diesel Engine), Jump Starter and Power Bank with LED Flashlight, IP67​

This is on special offer on Amazon.

This has been a very useful post and there has been some great advice given for which I thank you all. I have been doing very short journeys every day and it occurred to me that the car might benefit from a decent run to charge the battery. Mission accomplished and the mpg looks much healthier!

I also decided to buy the jump start battery for “just in case”. If the boot doesn’t open, then it seems you can get to a terminal via the fuse box to boot the car up.

Thank you again for your kind ideas, I already have a head torch so money saved on that one!

 

[Big Ears]

Car Jump Starter, FLYLINKTECH 2000A Peak 13200mAh Portable Car Battery Jump Starter Booster (up to 6.0L Gas Or 5.0L Diesel Engine), Jump Starter and Power Bank with LED Flashlight, IP67​

This is on special offer on Amazon.

This has been a very useful post and there has been some great advice given for which I thank you all. I have been doing very short journeys every day and it occurred to me that the car might benefit from a decent run to charge the battery. Mission accomplished and the mpg looks much healthier!

I also decided to buy the jump start battery for “just in case”. If the boot doesn’t open, then it seems you can get to a terminal via the fuse box to boot the car up.

Thank you again for your kind ideas, I already have a head torch so money saved on that one!

Check the reviews on these things.

 

[TonyD 2024]

Reviews at 4.4 out of 5 over 747 response which isn’t bad. I will hopefully never have to use it, but a flashlight will come in handy if my head torch runs out!

 

[Big Ears]

I went with this one, they do a 2000a one for a tiny amount less, the prices are up and down like a woman of ill-reputes drawers so a timed purchase is good, not sure if that too has a torch but this is well specced though.

https://amzn.eu/d/2kXlO6d

 

[Joningy]

We are all hoping never have to use the jump start I reckon Tony. but I would much prefer to start an EV than a 5 or 6 litre car with one of those things, I've previously mentioned, the one I bought had a similar spec, but refused to turn over my Hyundai Tucson a couple of months ago when the battery died, that was 2 litres, so I'm extremely sceptical about claims made with these tiny devices.

 

[Big Ears]

I don't know what the EV battery voltage is and can't get access to it anyway, there is a volt meter on the dash I think I saw 14v on that but I had the engine running at that point so it may have been the starter battery being charged, I can't find the info I need.

I assume that if left for weeks the EV battery will hopefully keep the starter battery topped up, but not found that info either, I have asked a week ago at the dealers but nothing forthcoming from them either

 

[siteguru]

14V absolutely is the nominal 12V battery under charge.

In the MG4 the HV battery pack is nominally 400V, but I doubt the MG3+ battery is that voltage ... at a guess I'd say it is a nominal 48V system.

 

[Coulomb]

14V absolutely is the nominal 12V battery under charge.

To me, the nominal voltage is the one that gives it its name. It's the term we use to describe it. You will find "12 V" on the battery label, so to me that's its nominal voltage, regardless of its actual voltage under charge or any other condition.

It's certainly normal (not the same as nominal) to see 14 V or even slightly higher when being charged by a powerful source such as an alternator or DC-DC converter.

 

[siteguru]

Maybe you've misinterpreted my post? That's what I was saying ... 12V is the nominal battery voltage (for the battery under the bonnet/hood). In ICE cars the alternator causes the voltage at the battery terminals to read circa 14V, so I'm presuming in an EV the charging circuit is emulating the same function - bear in mind that in the MG cars the 12V battery is a standard one as found in an ICE car, even though an EV doesn't need the CCA specification.

 

[Big Ears]

Oh okay, how do we do that then? how do we even check the High-voltage battery to determine it charge level?

I did search the manual and it only mentions volts once and that is here.